Do you have any idea why Nana continued to dehumanize Shigaraki after learning that her murderer groomed him to be evil?
I think what a lot of people don't get with the whole "you might have to kill Shigaraki" thing is that Nana (and Torino by extension) aren't removing his autonomy when they say that.
At a certain point, it doesn't matter who's fault it is for turning someone into a villian. The entire purpose of Deku's rematch with Muscular is to make it clear that some people aren't going to stop destroying things and hurting people by talking about their abusive dad or their dead mom or whatever tragic circumstances led them here.
Tomura has also reached a similar point, refusing to compromise on his goal to destroy the causes of his suffering. That includes both All for One and the society that left him to rot. AFO meticulously planned a lot of Tenko’s misfortune but he didn't manipulate the strangers that passed him on the street. He didn't create that feeling of abandonment, only pointed it in the wrong direction. Tomura not making this choice for himself because he was groomed genuinely doesn't matter anymore because these are his choices now. He's shown multiple times to understand that AFO is using him against All Might and for his own ends, but he doesn't try to get away and turn to the heroes; he tries to get away so he can be a villian on his own terms.
It's easy to forget, since he's the protagonist, but Izuku's optimism about saving the crying boy at the core of who Tomura is, is a massive outlier and his leaps in logic aren't always solid. Nana is very callous when she talks about it, my personal guess is it's because she's still not fully willing to face her sense of guilt that her attempt to save her son from AFO led to the creation and abuse of Tomura. But she is still correct, regardless of what pain lingers inside Tomura, it is too late to help him.
Even when Izuku reaches his hand to Tenko when he is at his absolute most vulnerable, he maintains his loyalty to the league. He's been cast from society for so long that there's nothing that can convince him to stop. He would rather die destorying than live in a world he couldn't change.
In fact most of the major deaths in the league maintain this theme. Twice would rather die fighting than let his friends get screwed over by his own shortcomings again. Toga would rather die a free girl that lived as she pleased than fall in line with society. The heroes didn't choose to kill any of them from the outset, they could have gone quietly at any time to save their own skin, but their cause was more important.
This is what Nana means when she maintains that Shigaraki dying is a possiblity that needs to stay on the table, because he won't allow anything that isn't victory or death. It's tragic but I think it helps convey the message that the ending wants to, that change in how we treat the strange and disparaged doesn't just need to happen, but it needs to happen sooner. You can't just wait until a villian tries to blow up the country to try and help them, you need to give that kindness to everyone before those villians can even exist.
Sorry this got off topic because I really hate how people simplify the ending as bad writing when to me it seems really obvious that Horikoshi was going for a bittersweet ending with the league. They did destroy society but didn't live long enough to see what it rebuit into because even trust in one person trying to save them isn't enough when you're this far gone.
Either way, Horikoshi's writing style leads to a lot of necessary intuiting about the characters to actually understand the motivations for how they act. I think Nana ends up being a particularly rough victim of this because her stoicism and blunt attitude is very obviously supposed to be a front.
She opened herself up to others and All for One killed her loved ones, so she guards her heart the best she can to avoid showing more weaknesses. She does feel like a failure to her legacy; she never fully believes she made the right choices regarding Kotaro or Tomura. But if she cracks then not only is she done but AFO will make it his duty to twist the knife and hurt the people she cried for too.
She's a contrast to Izuku serving as a cautionary what-if scenario if Izuku's big open heart just makes more people targets by association (which it does!). She's not entirely right when she calls Tomura a monster but she isn't being entirely honest with herself either.